This affects module interfaces, interface generation in sourcekitd, and
diagnostics. Also fixes a fixit that was assuming the 'OSX' spelling when
computing the source range to replace.
Resolves rdar://problem/64667960
Completely mechanical changes:
- Explicit @objc in a few places
- Some imported APIs changed
- For the mix-and-match tests, just test version 4/5 instead of 3/4